A T-LED (Tube LED) is an LED based illumination device, where the LEDs are positioned as a linear lighting system in such way that the LEDs are carried by a mechanical carrier that can easily be mounted like a fluorescent tube in an office luminaire or similar. It is advantageous if the T-LED emits light uniformly. In current T-LEDs, the uniformity may be achieved by surrounding the LED board with a diffuser. A disadvantage of the diffuser solution is that the part of the light that is scattered back from the diffuser towards the LEDs requires multiple reflections before it can leave the T-LED towards the room, which reduces the system efficiency.
FIG. 1a shows a view from above of a light emitting arrangement comprising three light emitting elements, e.g. LEDs, 10a-c mounted on a PCB 16. FIG. 1b shows how the light emitting elements 10a-c emits a light 14. The light 14 emitted by this light emitting arrangement may be perceived as spottiness by an observer of the system since only three distinct light spots will be seen. Such an arrangement may be annoying for the observer. A solution to this problem would be to increase the number of light emitting elements 10a-c mounted on the PCB 16. This would lead to increased cost for the system which is not desirable. A further solution is to install the PCB and the light emitting elements 10a-c in a transparent tube 22 with an inner surface 220 made diffusing, as visualized in FIG. 2. A portion 20a, 20b of the light 14 emitted by the light emitting elements 10a-c is scattered back by the diffusing inside 220 of the diffuser 22 towards the PCB 16 where it is scattered back again to illuminate the diffuser 22 (indicated by 21a, 21b). As a result, a more homogeneous illumination is obtained at the diffusers level. Part of the light penetrating the diffuser 22 is scattered forward 18 by the diffusing inside 220 and consequently spread out leading to a more homogeneous illumination as perceived by the observer.
A disadvantage is that the part of the light that is scattered back 20a-b from the diffuser 22 towards the PCB 16 requires additional reflections before it can leave the light emitting arrangement towards for example a room. Since each reflection will result in some absorption, the system efficiency is reduced. Current T-LED systems, e.g. as described in FIG. 2, typically have 90% light efficiency, 10% light loss, while the spottiness is on the limit of acceptability, and for some people failing their acceptability level.
US20120106144 proposes a LED tube lamp that includes a heat sink, a LED substrate, a pair of connectors, and a cover fixed to the heat sink. The cover includes a first cover and a second cover, at least one optical lens is arranged on the first cover, the at least one optical lens comprises a concave lens and reflective lenses arranged on both sides of the concave lens. The concave lens is configured to refract light beams from the LEDs in a forward direction or in an approximate forward direction, and the reflective lenses are configured to reflect light beams from the LEDs in a lateral direction.